It may not be rational, but the most often given reason by those who speculate about this subject is that the Beeb does not want to make it easy for ITV (or others) to schedule their competing programs favorably against the BBC's programming.

Well, when the BBC aired the 4th series of "Doctor Who" there was about a month "lag time" from then to when it aired in the US (on Sci-Fi).

That is to be expected, but I wish could get a simultaneous broadcast like they did for "The Five Doctors" when it came out. They did that specially for the 20th anniversary - so why can't they do it now?